This invention relates to a constant-velocity universal joint for a propeller shaft used at a coupling portion of a propeller shaft for transmitting a driving force in the back-and-forth direction of an automobile body.
Such a constant-velocity joint for a propeller shaft, and a constant-velocity joint for a drive shaft provided near a wheel and used at a coupling portion of a drive shaft for transmitting the rotation force from a differential gear to the wheel are typical constant-velocity joints for automobiles.
Since a propeller shaft is a rather heavy object and rotates at a high speed compared with a drive shaft, a load on the constant-velocity joint is large and the accuracy of its motion influences the degree of perfection of the propeller shaft.
A cross-groove type constant-velocity joint is known as a constant-velocity joint used for such a propeller shaft. It has a general structure as shown in FIGS. 1 and 2, in which a plurality of grooves (also referred to as tracks) formed in the outer peripheral surface of an inner ring 1 and the inner peripheral surface of an outer ring 2 extend in such directions as to cross each other (in a torsional relationship in a three-dimensional view) with balls 5 mounted at portions where grooves 3 of the inner ring 1 and opposing grooves 4 of the outer ring 2 cross each other. The balls 5 are rotatably retained by a cage 6.
In such a constant-velocity joint, a lubricating grease is sealed to prevent abnormal wear at portions that are liable to wear due to the rotation of the shaft and to increase durability, thereby maintaining the rotational resistance at a low level.
Various designs are proposed to cross-groove type constant-velocity joints for propeller shafts. Ones are preferable which are designed such that there will be no internal clearances, particularly no clearance in the pitch circle (PCD) direction (referred generally to as PCD clearance) so that they rotate smoothly without producing vibration or abnormal sound especially during high-speed rotation. In particular, ones structured such that the PCD clearance is negative and a preload is applied are used in applications where high-speed rotation is required.
Since such conventional constant-velocity joints for propeller shafts are used in applications where they rotate at a high speed exceeding 2000 rpm, the grease may be heated to high temperature and deteriorate. If this happens, the frictional surface may not be lubricated sufficiently.
Also, if a grease containing organic lead compound as an extreme-pressure agent is sealed in a constant-velocity joint for a propeller shaft, deterioration in quality of such a grease can be avoided. But in view of a bad influence of the disposed grease on the biosphere, using an organic lead compound as an additive should be avoided if possible.
An object of this invention is to provide a constant-velocity joint for a propeller shaft which can be used with good durability without adding any organic lead compound, which is high in the effect of suppressing temperature rise of grease (so-called temperature-rise suppressing effect) especially during high-speed rotation, and which can prevent quality deterioration of the grease.
Another object of this invention is to provide a cross-groove type constant-velocity joint for a propeller shaft in which the PCD clearance is negative and a preload is imparted, and which is high in the temperature-rise suppressing effect.